Many substances such as frozen food, medicine, or explosives, must be kept below a predetermined temperature because, if the temperature of the substances rises above a predetermined level for a predetermined period of time, the substances can deteriorate sharply in quality and become dangerous. This is particularly true of food and medicines. In the case of food, spoilage is quite rapid when the temperature of the food is at or near the melting point of water. However, circumstances can arise where, for example, because of a power failure in a refrigerating device, the temperature of the frozen food rises above the predetermined level for a predetermined period of time, then after the power is restored, the temperature again falls. In the meantime there might have been a serious deterioration in the quality of the food or the safety of the material that has been subjected to a rising temperature. In such a situation, it is important to be able to know whether or not the temperature of the substance has exceeded a predetermined level for the predetermined period of time.
To prevent this from happening, a great many thaw indicators have been devised. Those previously constructed usually involved a device of some sort which broke upon freezing and when the temperature of the device rose above a predetermined level, the contents of the indicators caused a visible stain or color change, which indicated the temperatures of the substance exceeded the limits set for it.
There were, however, certain problems associated with thaw indicators previously constructed. For one thing, they were not made of edible substances. This created safety problems when the packages were used in places, such as the home, where children could be attracted to the indicator and eat it, particularly if the color change resembled the color of food or candy.
In addition, previous thaw indicators were often attached to the package in such a way that in the event that the temperature of the package rose above the proscribed limit, causing the indicator to function, a merchant might be able to simply remove the thaw indicator and replace it with one which had not yet functioned in order to sell the package.
Accordingly, an important object of this invention is to provide a thaw indicator which is made entirely from edible and non-toxic substances.
A further object of this invention is to provide a thaw indicator for use on a package of frozen foods to indicate that the package has been subject to an unsafe temperature for a predetermined period of time.
Yet another object of this invention is to provide a thaw indicator which cannot be reset after it functions.
Still a further object of this invention is to provide a thaw indicator which can be made very tiny, so it can be mounted inside packages and medicine boxes.